Lactic acid has a molecular structure CH.sub.3 CH(OH)COOH. It is the simplest optically active compound having D-and L-optical isomers as enantiomer. ##STR1## The two compounds differ only in optical activity while their chemical properties are completely identical. Lactic acid exists in nature as L-form. Microbial fermentation. produces only L-lactic acid. The production of D-lactic acid is very difficult. Since the preparation of some optically active pharmaceuticals needs D-lactic acid as a starting material which cannot be substituted by L-lactic acid, the preparation of D-lactic acid is very important.
Conventional processes for the preparation of D-lactic acid in laboratories include the followings: